Grates in the combustion chamber of a combustion furnace are typically comprised of a plurality of rows of grate bars which form at least a portion of the floor of the combustion chamber. The rows of grate bars are typically laid so that each row at least partially overlaps another row in the same way that rows of roofing shingles or tiles overlap one another.
Various different types of grate bars are used in waste incinerators. Larger installations frequently utilize clamped or pressed grates.
However, when the grate bars used are not clamped or pressed into position, the grate bars generally are positioned in rows in the combustion chamber so that they have a lateral clearance for thermal expansion, as disclosed in German Patent Document No. A1-26 52 475. The expansion clearance for these grate bars is generally distributed as spaces or air gaps in a non-uniform manner over the entire width of the grate. Along the sides of the grate proximate to the walls of the combustion chamber, there is generally an additional space or air gap to provide further expansion clearance. In these conventional grates, the air gap between the grate bars provides access for an uncontrolled flow of air to the material being incinerated in the combustion chamber of the furnace.
In an attempt to solve this problem, adjacent grate bars have been connected by individual connection elements to form a single gap-free incineration grate surface as described in German Patent Document A1-38 13 441. However, if the temperature of the material being incinerated and the resulting temperature of the grate bars is too high, the grate bars produce stresses which are higher than the connection elements can withstand, and distortions or breakage of the grate bars frequently results.
In addition to the problems of the uncontrolled supply of combustion air, the presence of air gaps or slots between the grate bars provides a location where material being incinerated can enter and pass through, thereby leading to an accumulation of incinerated ash under the grate and, in addition, exposing the various structural parts below the grate to elevated temperatures. Also, materials that melt at a low temperature, such as non-ferrous metal waste and steel scrap, can flow through and become wedged in these gaps, thereby restricting the mobility of the grate bars.
It is also desirable to provide a means for feeding the material into the combustion chamber of the furnace, and for shifting the material in the combustion chamber to ensure that the material is incinerated completely and efficiently. Typically, the feeding and shifting of the incinerator material is accomplished by providing that one or more of the rows of grate bars can slide as an entire row over a stationary, underlying row of grate bars. Swiss Patent Document No. 585,372 discloses such a feed grate for a waste incineration furnace in which rows of grate bars, which extend transverse to the grate, are moveable in the direction of feed of the grate.
As pointed out above, in incinerators used in large plants, frequently the grate bars of the grates are clamped or pressed into position. Swiss Patent Document No. 585,372 additionally describes a grate in which each transverse row of grate bars is provided with a clamping device which operates under spring action. The clamping device is connected to each of the two outermost grate bars of each transverse row of grate bars and urges the two outermost grate bars, and thus the intervening grate bars, toward the center of the transverse row so that any gaps between the grate bars are eliminated. However, because of the large number of moving parts, this spring tension design is usually unable to withstand prolonged use under the hostile conditions present in a high temperature furnace. Additionally, such a device is expensive to manufacture and is not easily controllable.
Swiss Patent Document No. 619,764 discloses an alternative design in which spring loaded spreader elements are positioned between certain of the grate bars. European Patent Document No. B1-0 165 432 discloses a mechanism using plate springs in which grate bars are resiliently connected to one another by a tie rod. However, plate springs cannot reliably provide uniform spring forces over the large temperature range present in the furnace and the high pressures resulting from the thermal expansion of the grate bars and the tie rod. In addition, the springs and various moveable parts must be exposed to the high temperatures of the combustion chamber and to the material being incinerated. Finally, adjustment and control of the tension and repair of the spreader devices can only be accomplished once the grate has fully cooled.